The objective of this project is the elucidation of the structural determinants leading to specific high-affinity binding of hCG to its receptor and the mechanism of ligand-mediated receptor activation. There are two major aims, namely 1) Delineation of hormone and receptor binding determinants and 2) Elucidating the mechanism of receptor activation and transmembrane signaling. Site-directed mutagenesis of alpha subunit, hCG-beta, and LH/CG receptor will be used to test the hypothesis that discrete regions of hCG and of receptor are responsible for high-affinity binding. The investigations will exploit a hormone-receptor polypeptide fusion technique to construct a putative mini-gonadotropin. Receptor mutagenesis studies will be utilized to test the hypothesis that transmembrane helices 6 and 7 contain amino acid residues that are important in signaling but not ligand binding.